The cathode ray tube (CRT) is the predominant display device for electronic systems such as computers and televisions, notwithstanding its many shortcomings. Among these shortcomings are the large spatial volume ("footprint") occupied by a CRT, the high power supply voltages needed for the tube, and the relatively short lifetime of the tube.
In recognition of these shortcomings, the art has heretofore proposed many alternative solid state displays, such as light emitting diodes (LED), liquid crystal displays (LCD), and electroluminescent (EL) displays. However, solid state displays have their own limitations which prevent replacing a CRT for many applications. The present invention is directed to overcoming the performance limitations of electroluminescent displays.
As is well known to those having skill in the art, electroluminescent displays are formed by applying an electric field across an electroluminescent material, typically a phosphor, in sufficient magnitude to cause avalanche breakdown of the phosphor. The light generated by recombination of electron-hole pairs produced by the breakdown can be tuned in wavelength by the addition of various impurity ions to the phosphor. Arrays of electroluminescent elements may be used to create television screens, computer graphic displays and a wide range of other display devices.
Most electroluminescent display devices are two terminal devices, having a pair of electrodes on opposite faces of the electroluminescent layer (phosphor). Often, the phosphor is formed on one face of a semiconductor substrate, so that the semiconductor substrate can act as a carrier injector, typically an electron injector, upon application of an appropriate bias voltage between the exposed face of the phosphor and the exposed face of the semiconductor substrate. Examples of such two-terminal electroluminescent devices may be found in U.S. Pat. Nos. 4,486,499 to Morimoto entitled Electroluminescent Device and 4,720,432 to VanSlyke et al. entitled Electroluminescent Device With Organic Luminescent Medium.
A major limitation which has prevented widespread use of electroluminescent displays is their low intensity or brightness level. This low intensity results from an inherent limitation on the number of carriers, typically electrons, which can be injected into the phosphor from the semiconductor substrate, due to space charge at the interface of the phosphor and semiconductor substrate. In particular, the bias voltage across the semiconductor substrate produces electrons in the substrate, which are accelerated towards the phosphor layer by the bias voltage. Many of the electrons have sufficient energy to overcome the barrier presented by the phosphor layer, and enter the phosphor layer to cause avalanche breakdown and recombination. However, the electrons which do not have sufficient energy to overcome the barrier of the phosphor layer, form a space charge region in the semiconductor substrate. This space charge region forms an additional barrier to penetration by the electrons which are accelerating towards the phosphor layer, so that the number of electrons which are injected into the phosphor becomes self limiting. Since the number of electrons which can overcome the barrier are self limiting, the ultimate intensity which can be produced is also limited.
The art has attempted to overcome the space charge limitation by providing an alternating current (AC) bias across the electroluminescent display. Space charge builds up during, for example, the positive half of the alternating current bias, but decays during, for example, the negative half of the AC bias. See for example, U.S. Pat. Nos. 3,806,759 to Kabaservice et al. entitled Electroluminescent Cell With Integrated Switching Control; 4,603,280 to Pankove entitled Electroluminescent Device Excited by Tunneling Electrons; 4,613,793 to Panicker entitled Light Emission Enhancing Dielectric Layer for EL Panel; and 4,751,427 to Barrow et al. entitled Thin Film Electroluminescent Device. Unfortunately, the need for AC bias complicates the driving circuits of the electroluminescent display, and also limits the total amount of light which can be produced by the electroluminescent device, since light is not produced at least one half of the time. Moreover, during the half of the cycle when light is produced, space charge limitations still exist.
The art has also suggested a three terminal electroluminescent device which adds a "gate" electrode to control or modulate the brightness of the electroluminescent display. See for example, U.S. Pat. No. 4,554,485 to Yamada entitled Solid State Image Display Device. Yamada describes an electroluminescent display including an electron injection region of high impurity density on one face of the semiconductor substrate, and a phosphor region on the opposite face of the semiconductor substrate. The amount of electrons injected into the phosphor is controlled by a drive control region provided in the one face of the semiconductor substrate opposite the phosphor, and aligned between the electron injection region and the phosphor. The drive control region (also called a "gate") controls the amount of carriers injected from the semiconductor substrate by forming a depletion region across the electron injection region. The area of the semiconductor substrate through which electrons can travel is reduced by the depletion region, and the current is thereby reduced.
Although the Yamada device provides brightness control, by using a gate region in the face of the semiconductor substrate opposite the phosphor layer, this device is still susceptible to space charge limitations. During device operation, the face of the semiconductor substrate which lies under the phosphor becomes saturated with low energy electrons. These electrons, which do not have sufficient energy to overcome the phosphor barrier, form a negative space charge region. This negative charge effectively increases the height of the barrier, by repelling electrons that accelerate towards the barrier, leading to a rapid saturation in the current flowing through the phosphor. Since the light output of the device is proportional to the current in the phosphor, the device brightness is limited.